1. Field of the Invention
The present invention relates to spherical silica particles useful as, for example, a filler in epoxy resin sealants for integrated circuit (IC) devices, an internal additive for toners, an abrasive, and a reinforcing filler for rubber. The invention also relates to a method of producing such particles.
2. Prior Art
Known methods for producing silica particles include the flame hydrolysis of silane compounds and the flame pyrolysis of alkoxysilane compounds.
Production by the flame hydrolysis of a silane compound generally involves heating and vaporizing a chlorosilane such as silicon tetrachloride (SiCl4), feeding the vapor to a burner, and hydrolyzing the vapor in an oxyhydrogen flame to form silica particles. However, the unavoidable presence of chlorine from the silane compound in the resulting silica particles, coupled with the excessively small size of the particles and their tendency to agglomerate into secondary particles, makes the silica particles obtained by such a process unsuitable for use as a filler in epoxy resin sealants for IC devices, as an internal additive in toners, or as an abrasive.
Production by the flame pyrolysis of an alkoxysilane compound generally involves heating and vaporizing a compound such as tetramethoxysilane (Si(OCH3)4), feeding the vapor to a burner, and pyrolyzing the vapor in an oxyhydrogen flame to form silica particles. Although halogen-free, high-purity silica particles can be obtained in this way, the stoichiometric ratio of the silica formed to the silane starting material is low. As a result, the silica concentration within the flame is low, and so collisions and coalescent growth of the silica are infrequent, discouraging growth in the size of silica particles formed by the reaction into larger particles. Particles larger than 500 nm cannot be obtained by this process. An additional disadvantage is the high price of alkoxysilanes, which increases the cost of production.
It is therefore an object of the invention to provide spherical silica particles and a method for their production which overcomes the disadvantages of such prior art processes.
We have conducted investigations on producing high-purity spherical silica particles which contain substantially no halogen and have a suitable particle size by the combustion of a non-halogenated siloxane starting material. As a result, we have found that spherical, non-crystalline silica particles which contain substantially no halogen and have a content of metallic impurities other than silicon of not more than 1 ppm, a particle size of 10 nm to 10 xcexcm and a specific surface area of 3 to 300 m2/g can be cost-effectively produced by the oxidative combustion of a non-halogenated siloxane in a flame provided that, based on the siloxane, a combustion-assisting gas and a combustion-supporting gas fed to a burner, the siloxane and the combustion-assisting gas when burned have an adiabatic flame temperature within a range of 1,600 to 5,600xc2x0 C.
Accordingly, the invention provides spherical, non-crystalline silica particles which are made by burning a non-halogenated siloxane starting material, are substantially halogen-free, and have a content of metallic impurities other than silicon of not more than 1 ppm, a particle size of 10 nm to 10 xcexcm and a specific surface area of 3 to 300 m2/g.
The invention also provides a method of producing such spherical silica particles by subjecting the non-halogenated siloxane to oxidative combustion in a flame. In this method, based on the siloxane, a combustion-assisting gas and a combustion-supporting gas fed to a burner, the siloxane and the combustion-assisting gas when burned have an adiabatic flame temperature within a range of 1,600xc2x0 C. to 5,600xc2x0 C. According to one preferred embodiment of the method of the invention, combustion is carried out by feeding the siloxane to the burner in liquid form and atomizing it with a nozzle mounted on the burner. According to another preferred embodiment, combustion is carried out by heating the siloxane to form a vapor and feeding the vapor to the burner.